Method of making charge storage electrodes for charge storage tubes



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Filed Jan. l5, 1958 FOR CHARGE STORAGE TUBES M. F. TOOHIG ETAL METHOD OF MAKING CHARGE STORAGE ELECTRODES Jan. 2, 1962 United States Patent @dice 3,@15585 Patented Jan. 2., 1.952

3,015,586 METHOD F MAKING CHARGE STGRAGE ELEC- E'RBES FR CHARGE STGRAGE TUBES Michael F. Toohig, Fort Wayne, and Cyril L. Day, Huntington, 1nd., assignors to International Telephone and Telegraph Corporation Filed Jan. 1S, 1958, Ser. No. 709,072 6 Claims. (Cl. 117-261) This invention relates to charge storage tubes and more particularly to the charge storage electrode of such tubes and to the method of making such charge storage electrodes.

A common form of charge storage tube is an electrical output tube generally referred to as a barrier-grid storage tube; barrier-grid storage tubes are often used in such devices as binary computers for storing and reading-out yes-no answers. These tubes, which are the cathode ray type, are well known in the art, as shown for example in Patent 2,538,836 of January 23, 1957, to A. S. Jensen. Barrier-grid storage tubes conventionally include an electron gun assembly including a cathode heated by a suitable larnent, a control grid and an accelerating anode positioned within an elongated envelope at one end thereof. Conventional detlecting and focusing elements are provided for causing the electron beam produced by the electron gun to scan a charge storage electrode or target positioned within the envelope at the other end thereof. The charge storage electrode comprises a line-mesh metal grid or screen arranged on one side of a dielectric sheet, which in turn is arranged on one side of an electrically conductive surface or backing plate.

The high velocity electron beam from the electron gun is caused to scan the charge storage electrode thereby providing secondary emission, ie., emission of secondary electrons from the dielectric surface of the charge storage electrode, such that the secondary emission ratio is greater than unity. Each discrete element of the charge storage surface in essence lforms a separate capacitor with the conductive baclc'ng and thus may be charged positively or negatively by the electron beam, depending upon the polarity of the input signal applied to the conductive backing surface; these charges may subsequently be read out by a subsequent scanning of the charge storage electrode by the high velocity electron beam.

ln electrical output charge storage tubes of the barriergrid type, the most widely used charge storage electrode has been provided with a dielectric material layer formed of a ilat sheet of mica about 25 microns thick. Developments in the barrier-grid storage tube held have vindicated the desirability of providing a charge storage electrode having an essentially spherical curvature, the dielectric layer of which should be 20 through 50 microns thick, such layer having a glass-like surface, being well bonded to the electrically conducting surface or substrate, with the entire assembly being capable of being processed from room temperature to the temperatures used in processing the completed tube, i.e., generally in the order of 400 C. The glass-like dielectric surface is deemed desirable since, if a porous surface is presented by the dielectric layer, impingement of the high velocity Writing electron beam thereon will cause charging of the pores of the dielectric layer, thus producing a residual image which may not be completely erased during the next erasing operation. Increased thickness of the dielectric layer is sometimes desirable in order to reduce the capacitance of the charge storage electrode, and it is further desirable that the dielectric layer be bonded to the electrically-conducting substrate. `lt is desirable to provide a sphericallycurved charge storage electrode assembly so that the surface thereof is always normal to the electron beam; this eliminates a source of serious signal shading found in tubes with ilat targets. The provision of such a sphericallycurved charge storage electrode assembly was extremely difficult, if not impossible, when employing sheet mica as the dielectric layer. lt is further necessary that the charge storage electrode assembly be capable of withstanding the elevated temperatures to which the completed tube is subjected during processing; the completed tube is conventionally heated to a temperature in the order of 40G C. during itsvevacuation in order to drive out gasses; this is conventionally referred to as out-gassing.

=lt has been proposed that the charge storage electrode for charge storage tubes be produced by evaporating a film of a dielectric material having the physical properties of glass, such as silicon monoxide, onto the electrically conducting substrate, which may be either a metal member or a glass member having one surface coated with a thin conductive hlm. Evaporation techniques for producing thin iilms (l micron or less) are now well known in the art. yIt has been found, however, that 25 micron thick dielectric layers produced by using conventional evaporating techniques crazed badly when heated totemperatures on the order of 400 C., i.e., many hairline cracks were formed in the surface of the evaporated dielectric material. lt is therefore desirable to provide a process for producing by evaporation techniques a thick dielectric layer on an electrically-conducting substrate, such dielectric layer having 'a glassy surface, being Well bonded to the substrate, and most importantly not being subject to crazing when heated to elevated temperatures. It isl further desirable that such a process be 'applicable to the forming of essentially uniform dielectric layers upon a spherically-curved, electrically-conducting substrate.

We have found that charge storage electrodes having the above set forth characteristics can be produced by initially preheating the substrate to a temperature above the temperature at which the entire tube will be later processed, evaporating the dielectric material onto the sur-l face of the substrate in a vacuum, and then controlling the cooling of the substrate with the dielectric material layer thereon so that the substrate always keeps the dielectric material layer under compression. More specifically, in accordance with our invention, the relative cooling rates of the substrate and the dielectric material layer thereon are controlled so that the dielectric material layer cools more slowly than the substrate, i.e., is at a temperature higher than the substrate during cooling. It has been found that by maintaining the dielectric material layer under compression, the charge storage electrode assembly may subsequently be heated to over 400 C. without crazing of the surface of the dielectric material layer. With this process therefore, dielectric material having the physical properties of glass, such as silicon monoxide, may be utilized thus providing 'a resultant glassy surface, the layer being well bonded to and in intimate contact with the electrically-conducting surface of the substrate member. ln addition, the dielectric material layer may be thicker than has been previously possible using evaporation techniques and may be formed on the inner surface of a spherically-curved subst-rate member.

It is therefore an object of our invention to provide an improved method of making a charge storage electrode for a charge storage tube.

Another object of our invention is to provide an improved method of producing a relatively thick layer of dielectric material having the physical characteristics of glass on an electrically conducting surface of `a substrate member, the resulting dielectric material layer presenting a glassy surface, being Well bonded to and in intimate contact with the electrically conducting surface of the substrate, and being capable of subsequent heating to elevated temperatures without surface crazing.

. A further object of our invention is to provide an improved method of producing a relatively thick layer of dielectric material having the physical properties of glass on a substrate in which the vdielectric material layer is maintained in compression by the substrate member.

Yet another object of our invention is to provide an improved charge storage electrode for charge storage tubes.

A still further object of our invention is to provide an improved charge storage electrode for charge storage tubes in which the dielectric material layer is maintained in compression by the substrate member.

r[he above-mentioned and other features and objects of this invention and the marmer of attainingthem will become more apparentand the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings,rwherein: K

FG. l is a cross-sectional view of a typical barrier grid storage tube incorporating the improved charge storage electrode produced in accordance with our invention; and

FIG. 2 is ya cross-sectional view showing the apparatus for practicing our improved method. Y

Referring now to FIG. l, there is shown a typical barrier-grid storage tube, generally identified as 1, having an envelope 2 with a neck portion 3 at one end in which is disposed a conventional electron gun assembly 4; it will be understood that the electron gun assembly 4 for providing the high velocity electron beamY 5 includes conventional cathode, control grid and accelerating anode elements as is well known in the art. The high velocity electron beam 5 is caused to scan the'charge storage electrode 6 by means of conventional vertical and horizontal deflecting elements 7; vertical and horizontal deflecting elements 7 may either be internal electrostatic deiiecting plates or external magnetic deiiecting coils as is well known in the art. An internal conductive coating 8 formed on the interior surface of the envelopeV 2 is provided for accelerating high velocity electron beam S and for dei'lecting the low velocity secondary electrons into the .collector field.

rthe charge storage electrode 6 includes a backing or substrate member 9, shown here as being sphericallycurved or bowl-shaped; substrate member 9 may be formed of metal or alternatively of glass with an electri- Vcally'conducting surface formed thereon, as by the well known iridizing process. A layer its of fuzed dielectric material having the physical characteristics of glass, for example, silicon monoxide, is mounted on the inner conducting surface of the substrate member 9 and a ne mesh metal grid or screen 11 is in turn arranged abutting the inner surface of the dielectric layer 19 and is preferably fuzed thereto. A collecting electrode l2 is arranged within the envelope 2 intermediate the charge storage electrode 6 `andthe electron gun 4 and serves to collect the secondary electrons emitted from the surface of the dielectric layer lil responsive to impingement of the electron beam 5 thereon. It will be understood that other focusing and accelerating electrodes may also be provided within the element envelope 2 as is well known in the art. lt will also be understood that the barrier grid storage tube 1 shown here is for illustrative purposes and the exact mechanical construction and arrangement of the tube does not form a part of Vour invention. Y

Methods ofY making the bowl-shaped substrate member 9, the bowl-Shaped line-mesh metal screen 1i, and of eecting the assembly of the complete charge storage electrode are more fully described in C. L. Days copending applications Serial Number 666,969, tiled June 20, 1957, Serial Number 668,643, tiled lune 28, 1957, Serial Number 668,671, also tiled .lune 28, 1957, and Serial -Number 681,497, led September 3, 1957, and in application Serial Number 672,754 of Michael E. Toohig, tiled .lune i8, 1957, all assigned to the assignee of the present application; our invention here is concerned entirely with the forming of the'layer of dielectric material on the substrate member in such a manner that the dielectric material layer is always maintained in compression by the substrate member.

Referring now to FG. 2, there is shown an apparatus for performing our invention comprising an annular metal support member 13 resting upon a base 14 with an annular metal ring l5 `resting on the top thereof. The bowl-shaped substrate member 9 is disposed on top of the ring member 15 with its spherically-curved portion coextensive with the annular opening 16 therein and facing the base'li. An `annular evaporating receptacle or boat i7 is provided formed of suitable high temperature metal, such as tantalum with electrical leads i8 extending through the base 14. The evaporating receptacle 17 is spaced from the substrate member 9 by a substantial distance as shown. An enclosure member i9 is arranged on top of the ring member 15 enclosing the substrate member 9 and a suitable heating coil 2li' is disposed therein having electrical leads Zl connected thereto and extending through the base A cooling uid coil Z2 is yalso disposed within the enclosure yi9 between the heating coil 29 and the outer surface of the substrate member 9 with cooling fluid'lines 23 connected thereto and extending through the base An outer enclosure 24, such as a bell jar is arranged over the entire assembly and in sealing contact with the base l@ with an exhaust line 2S extending through the base 14 and communicating with the interior 26; exhaustline 25 is adapted to be connected to suitable vacuum pump apparatus (not shown).

Assuming now that the substrate member 9 is formed entirely of metal and thus has a thermal expansion coetiicient greater than that of the dielectric material to be coated thereon, such as silicon monoxide, the metal substrate 9 is initially preheated by means of heater 20 until it is at a temperature slightly above the processing temperature of the tube l, for example to 460 C.; electrical leads 2l connected to resistance heating element 20 are adapted to be connected to a suitable external source of power (not shown) and suiicient voltage is applied across the leads 2 thereby to pass sucient current through the heating element 26 to provide the'requisite temperature. A suitable quantity of silicon rmonoxide is placed in the evaporating receptacle 17. Electrical leads 18 are adapted to be connected to a suitable source of power (not shown) and a suitable voltage is applied thereacross to pass sufiicient current through the receptacle i7 zto vaporize the silicon monoxide therein. The bell jar 24 is evacuated by means of vacuum line 25 during the evaporating step with the result that silicon monoxide molecules are projected with a cosine density `from the annular slot 27 in the evaporating receptacle 17 Onto the inner surface of the metal substrate 9 thereby to form the dielectric material layer, the heating of the substrate member 9 by means of heating element 2t) being continued during the evaporating step. When the required quantity of silicon monoxide has been evaporated from the receptacle 17 onto the inner surface of the metal substrate 9 to form the dielectric layer 1.4i of the desired thickness, by virtue 0f the fact that the metal substrate member 9 has a higher thermal expansion coeiicient than the silicon monoxide layer it?, it has been found satisfactory merely to terminate the heating of the substrate member 9 `by means of the heating element 20 and the heating of the evaporating receptacle 17; since the metal substrate member 9 Will then cool more rapidly than the silicon monoxide layer 1t), the metal substrate member 9 will at all times during cooling maintain the silicon monoxide layer 16 in comv pression so that in the completed assembly comprising the metal substrate member 9 and the silicon monoxide dielectric layer 10, the silicon monoxide layer 1t) is maintained in compression by the metal substrate member 9. 1t will be understood that after cooling has been completed, the bell jar 24 and enclosure i9 is removed in order to permit removal of the substrate member 9 with the silicon monoxide layer il) thereon and that the finemesh metal screen il is then assembled on the front surface of the silicon monoxide layer 10, for example, in accordance with our aforesaid co-pending applications.

ln cases where the substrate member 9 is formed of non-metallic material, for example, of glass with an iridized, electrically conductive surface, it will be apparent that the relatively thick glass substrate by virtue of having a lower thermal expansion coefficient than Ithe dielectric layer it? will normally cool more slowly than the dielectric layer 10 and thus normally tend to place the dielectric layer in tension rather than in compression. With such a non-metallic substrate member 9, therefore, it has been found expedient after the desired thickness of the dielectric layer lll has been obtained to terminate the heating of the substrate member 9 by the heater element 2d and then slowly decreasing the temperature of the evaporating receptacle 17 at such a rate that the silicon monoxide layer il@ is always at a higher temperature than the substrate member 9 so that the slowly cooling glass of substrate member 9 will at all times keep lthe silicon monoxide member 1i) under compression. In order that the evaporation of the silicon monoxide from the evaporating receptacle 17 may be terminated at any time with some silicon monoxide yet remaining in the receptacle 17, it may be found desirable to provide for the heating of the silicon monoxide layer 10 during the cooling step by means or an auxiliary ring-shaped heater 28 disposed outside of the line of iiight of the silicon monoxide vapor molectdes and yet in proximity to the silicon monoxide layer 1h. Heating element 23 is provided with electrical leads 29 to -be connected to a suitable source of power (not shown). Here again, when the desired thickness silicon monoxide layer l has been obtained, heating of the glass substrate 9 by means of the heater 2G is terminated, heating of the evaporating receptacle 17 is terminated, and the temperature of auxiliary heating element 2S is slowly decreased in order that at all times during the cooling step the silicon monoxide be at a temperature higher than the glass substrate 9 so that the glass substrate at all times maintains the silicon monoxide layer in compression. It may further be desirable to provide external cooling7 of the glass substrate 9 and in that case, after heating of the substrate 9 by means of the heating element Ztl has been terminated, a suitable cooling medium, such as liquid nitrogen, is caused to ow through the cooling coil 22 through lines 23 thereby speciiically to cool the outer surface of the glass substrate member 9 in order to accelerate its cooling and to assist in maintaining the silicon monoxide layer 10 in compression.

in an actual apparatus in accordance with FIG. 2 for forming the dielectric layer 10 on substrate 9 having a spherically-curved surface 41/2 inches in diameter, an evaporating receptacle 17 was provided having a diameter of l% inches with its annular slot 27 disposed 6 inches from `the upper surface of the ring member upon which the substrate member 9 was resting.

It will be seen that regardless of the material of which the substrate member 9 is formed, it is essential that the relative cooling rates of the substrate member 9 and the dielectric material layer 10 be controlled so that the dielectric material layer 10 cools more slowly, i.e., is always at a temperature higher than the substrate member during cooling, so that the substrate member 9 at all times maintains the dielectric material layer 10 in compression. lt will also be seen that while silicon monoxide has been described as the dielectric material of which the dielectric layer i0 is formed, other materials having the physical characteristics of glass may be equally advantageously employed. Layers of silicon monoxide from 2O to 40 microns thick have been formed on metal substrates of'Nilvar, Kovar, and Inconel and layers of silicon monoxide 2O to 30 microns thick have been provided on glass substrates in accordance with our invention. In every case, it was found that the resulting substrate and dielectric material layer could be subsequently heated to 499 C. without resulting crazing of the surface of the dielectric material layer. The resulting dielectric material layer presents a smooth glassy surface which is well bonded to the substrate member whether it 4be flat or spherically-curved as shown in the drawing. A charge storage electrode having its dielectric layer deposited or formed on the conducting substrate member in accordance with this invention may be heated to an elevated temperature during a subsequent processing of the completed tube without crazing of the surface of the dielectric layer.

It will now be seen that we have provided an improved method of making a charge storage electrode for a charge storage tube in which a dielectric layer having a glassy surface and being well bonded to the electrically conducting surface of the substrate member is provided, the surface of the dielectric layer not being subjected to crazing during subsequent heating of the completed tube. It will also be seen that we have provided an improved charge storage tube in which the dielectric layer is formed of material having the physical properties of glass and is held in compression by the substrate member thereby preventing crazing during subsequent heating of the completed tube.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention.

What is claimed is:

1. A method of forming a relatively thick layer of silicon monoxide on the inner surface of a bowl-shaped imperforate substrate member having at least its inner surface electrically conductive comprising the steps of: placing a charge of silicon monoxide in an evaporating receptacle; positioning said substrate member over said receptacle with its inner surface facing the same and spaced therefrom; positioning a container over said receptacle and substrate member and evacuating the same; preheating said substrate member to a temperature of approximately 400 C. while continuing said evacuation; heating said receptacle while continuing said evacuation and substrate heating thereby to evaporate said silicon monoxide onto the inner surface of said substrate member to form said layer; cooling said substrate member with said layer thereon while continuing said evacuation and controlling the relative rates of cooling of said substrate member and layer so that said layer cools more slowly than said substrate member whereby said substrate member always maintains said layer in compression; and terminating said evacuation and removing said substrate member with said layer thereon from said container.

2. The method of claim l wherein said cooling and control thereof is accomplished by slowly reducing the temperature of said layer at a rate such that said layer is always at a higher temperature than said substrate member.

3. The method of claim l wherein said substrate member is formed entirely of metal and wherein said cooling and control thereof is accomplished by simultaneously terminating said receptacle and substrate member heating and allowing said substrate member with the layer thereon to cool simultaneously while continuing said evacnation.

4. The method of claim l wherein said substrate member is formed of glass with a conductive coating on its inner surface and wherein said cooling and control thereof is accomplished by terminating said substrate heating and slowly decreasing the temperature of said receptacle at a rate such that said layer is always at a higher temperature than said substrate member during said cooling.

5. The method oclaim 1 wherein said substrate member is formed of glass with a conductive coating on its inner surface, and wherein said cooling and control thereof is accomplished by simultaneously terminating said substrate and receptacle heating, and heating said layer with an auxiliary heater and slowly decreasing the temperature thereof at a rate such that said layer is always at a higher temperature than said substrate member during said cooling.

6. The method of claim l wherein said substrate member is formed of glass with a conductive coating on its inner surface, and wherein said cooling and control there- References Sitesl in the iile of this patent UNHE STATES PATENTS 2,456,899 Strong Dec. 2l, 1948 2,546,623 Feb. 6, 1951 2,563,488 Aug. 7, 1951 2,819,419 De et 7, l958 2,901,649 Knight Aug. 25, 1959 ii .In 

